Since an exhaust gas of the internal combustion engine such as an automobile which use gasoline for fuel contains hazardous components such as hydrocarbon (THC), carbon monoxide (CO), and nitrogen oxide (NOx), it is necessary to simultaneously purify and exhaust each of the hazardous components using an oxidation-reduction reaction. For example, it is necessary to purify in such a manner that the hydrocarbon (THC) is converted into water and carbon dioxide by oxidation; the carbon monoxide (CO) is converted into the carbon dioxide by oxidation; and the nitrogen oxide (NOx) is converted into nitrogen by reduction.
As a catalyst (hereinafter, referred to as an “exhaust-gas-purification catalyst”) adapted to treat the exhaust gases emitted from such an internal combustion engine, a three way catalysis (TWC) capable of oxidizing and reducing CO, THC, and NOx has been used.
Three way catalysis has been generally used, in which a precious metal is supported on a refractory oxide porous material such as an alumina porous material having a high-specific surface area and the precious metal-supported porous material is supported on a substrate, for example, a monolithic substrate made of a refractory ceramic or metallic honeycomb structure, or on refractory particles. In general, such three way catalysis is attached in the form of a converter in the middle position of an exhaust pipe between an engine and a muffler.
It is important that a ratio of fuel to air (air-fuel ratio, A/F) is controlled to be close to a theoretical air-fuel ratio (stoichiometry) in the three way catalysis. That is, there is a tendency that a purification rate of HC and CO is high in a lean atmosphere in which the air-fuel ratio is larger than the theoretical air-fuel ratio and, conversely, a purification rate of NOx is higher in an excess fuel atmosphere, that is, a rich atmosphere in which the air-fuel ratio is smaller than the theoretical air-fuel ratio. Therefore, in order to obtain a high purification rate in all of three hazardous components, the air-fuel ratio is necessary to be controlled within a narrow range (referred to as a window) close to the theoretical air-fuel ratio (stoichiometry).
Therefore, in order to keep the composition of the exhaust gas to be close to the theoretical air-fuel ratio, a fuel supply is controlled by detecting an oxygen concentration contained in the exhaust gas using an oxygen sensor. Moreover, with respect to a catalyst, in order to absorb variation of the oxygen concentration contained in the exhaust gas and thus to improve exhaust gas purifying performance of the three way catalysis, a co-catalyst (referred to as an “OSC material”) having capacity of absorbing the oxygen when the oxygen concentration contained in the exhaust gas is high and of releasing the oxygen when the oxygen concentration contained in the exhaust gas is low, that is, an oxygen storage capacity (OSC) has been used.
As such an OSC material, ceria (cerium oxide, CeO2), a ceria-zirconia complex oxide, or the like is known. A grain growth of ceria in a high-temperature atmosphere facilitates a grain growth of a precious metal existing on the ceria, so that an OSC function of the ceria is decreased. Therefore, an oxide such as zirconia is added about 20% and a ceria-zirconia complex oxide is formed by forming a solid solution with such an oxide, so that it is possible to prevent the grain growth of ceria particles.
However, a pyrochlore-type ceria-zirconia complex oxide is known in addition to a fluorite-type ceria-zirconia complex oxide which is commonly used from the prior art.
Unlike the fluorite-type ceria-zirconia complex oxide, since the pyrochlore-type ceria-zirconia complex oxide is in a state where one of oxygen atoms is deficient and has a structure in which ceria atoms and zirconia atoms are regularly and alternately arranged, it is known to have excellent redox properties of Ce2O3CeO2 due to the change in oxygen concentration. That is, since the valence of Ce atoms contained in the oxide can be changed between a trivalent state and a tetravalent state, the valence of Ce atom is changed into the tetravalent state from the trivalent state to absorb the oxygen in an oxidizing atmosphere where a relatively large amount of O2 is contained and the valence of Ce atom is changed into the trivalent state from the tetravalent state to release the oxygen in a reducing atmosphere where a relatively large amount of CO and HC is contained. Therefore, when the pyrochlore-type oxide is added to the catalyst, variation in exhaust gas composition in the vicinity of catalyst components is reduced and thus the exhaust gas purification described above can more efficiently proceed. In addition, since heat is generated depending on oxygen absorption of the pyrochlore-type oxide, the generated heat can be used in the warm-up of the catalyst at the time of starting an engine, thereby expediting the start timing of catalytic activity. From these viewpoints, the pyrochlore-type oxide is useful as a co-catalyst of the exhaust-gas-purification catalyst.
With respect to such a pyrochlore-type ceria-zirconia complex oxide, the following inventions have been disclosed from the prior art.
For example, Patent Document JP 11-165067 A discloses a method of forming precipitates from a solution containing cerium (III) salt and zirconium (IV) salt using a coprecipitation method and holding the precipitates heated to a temperature of 800 to 1000° C. under an inert atmosphere or a non-oxidizing atmosphere. According to this method, the obtained complex oxide has an X-ray diffraction peak which belongs to a pyrochlore phase and high OSC is exhibited.
Patent Document JP 2005-170774 A discloses a complex oxide composed of CeO2 and ZrO2 that is characterized by having any one or more phases of a pyrochlore phase, κ-phase, or an intermediate phase of these both phases and having a specific surface area of 20 m2/g or more.
Patent Document WO2008/093471 A discloses a catalyst system to be used in an automobile exhaust gas purification apparatus, comprised of two or more exhaust-gas-purification catalysts including a first catalyst supported on an inorganic structural carrier and a second catalyst other than the first catalyst, characterized in that: the first catalyst is supported on a part of the inorganic structural carrier positioned at the upstream side when being arranged in an exhaust gas passage; on the other hand, the second catalyst is supported on a part of the inorganic structural carrier positioned at the downstream side when being arranged in an exhaust gas passage; and the crystal structure contains a cerium-zirconium complex oxide (A) having a pyrochlore phase.
Patent Document JP 2009-084061 A discloses a ceria-zirconia complex oxide containing complex oxides of ceria and zirconia characterized in that: a regular arrangement phase of a pyrochlore phase type is formed in the complex oxide by cerium ions and zirconium ions; and the regular arrangement phase of the pyrochlore phase type remaining after being heated at a temperature of 1000° C. for five hours in the atmosphere is 50% or more compared to that before the heating.
Patent Document JP 2012-110859 A discloses an exhaust-gas-purification catalyst formed by coating a honeycomb substrate with a catalyst layer, wherein, in the cross section vertical to the exhaust gas flow, the exhaust-gas-purification catalyst is formed by: coating a catalyst layer having a slow oxygen absorbing/releasing rate and containing an OSC material such as a ceria-zirconia complex oxide, which is formed with a regular arrangement phase of a pyrochlore phase type, on a center portion of the cross section; and coating a catalyst layer having a fast oxygen absorbing/releasing rate and containing an OSC material such as a general ceria-zirconia complex oxide on an outer periphery of the cross section.
The pyrochlore-type oxide was generally produced in such a manner that a mixture of cerium oxide and zirconium oxide is subjected to calcining, thereby producing a complex oxide CeZrO4, and then the complex oxide is subjected to reducing treatment to remove the oxygen.
As a method invention of producing a pyrochlore type oxide, for example, Patent Document JP 2003-246624 A discloses a method of producing a pyrochlore type oxide characterized in that: a mixed solution of a cerium compound, a zirconium compound, and a precious metal compound is prepared; after a reducing agent is added to the mixed solution, a solid substance containing cerium, zirconium, and a precious metal is prepared from the mixed solution; and then the solid substance is calcined in a non-oxidizing atmosphere.
In addition, Patent Document JP 2005-170774 A discloses a method of producing a complex oxide of CeO2 and ZrO2 characterized in that: an aqueous solution of a cerium compound and a zirconium compound and a precipitant aqueous solution are mixed with each other, thereby generating precipitates; and the generated precipitates is mixed with water and solvent forming an azeotropic mixture and is subjected to azeotropic dehydration treatment and then is calcined in a reducing atmosphere, thereby producing the oxide.